Highly Stable and Active Pt−Cu Oxygen Reduction Electrocatalysts Based on Mesoporous Graphitic Carbon Supports (original) (raw)
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Journal of Electroanalytical Chemistry, 1998
In situ X-ray absorption studies were done in 1 M HClO , with and without 0.3 M MeOH, on several well-defined carbon-supported Pt 4e lectrocatalysts with particle sizes in the range of 25 to 90 A. Data were obtained at several potentials in the range of 0.0 to 1.14 V vs. RHE. The results show that as the particle size is reduced below 50 A, the strength of adsorption of H, OH and C , moieties such as CO is 1 increased. The strong adsorption of OH explains the reduced specific activity for oxygen reduction on small particles. The reduced activity for methanol oxidation on the small particles is due to a combination of the increased strength of adsorption of both CO and OH. The strong adsorption of H at negative potentials on small Pt particles is sufficient to induce reconstruction and morphological changes in the Pt particles. Both XANES and EXAFS data on a 53 A particle at 0.84 V indicate that formation of PtOH is the rate determining step in the oxidation of methanol. All these affects are due to an increase in the number of Pt sites with low coordination on the small particles.
Quantitative XPS studies of a natural catalyst
Journal of Electron Spectroscopy and Related Phenomena, 1995
The surface characterization by X-ray photoelectron spectroscopy (XPS) of a natural mineral found in the Venezuelan Andes is reported. We present XPS studies for the natural mineral (sample ULA2) and for the mineral after it has served as a catalyst (sample ULA1) in a Fischer-Tropsch reaction for three months. The effect of different treatments on these samples is also investigated. Both samples exhibit a differential charge effect which is present once their surfaces have interacted with hydrogen. The quantitative XPS results allow us to advance the composition of these samples. The presence of a negatively charged hydrogen species in our samples is discussed based on our XPS results and supported by work function measurements.
In situ X-ray absorption spectroscopy and X-ray diffraction of fuel cell electrocatalysts
Journal of Power Sources, 2001
The utility of in situ X-ray absorption spectroscopy (XAS) in determining structural parameters, through analysis of the extended X-ray absorption ®ne structure (EXAFS), and electronic perturbations, through a white line analysis of the X-ray absorption near edge structure (XANES), is demonstrated for Pt/C, PtRu/C and PtMo/C fuel cell electrodes. The results provide veri®cation that the enhancement of CO tolerance of the alloy catalysts occurs via an intrinsic mechanism for the PtRu alloy, whilst a promotion mechanism is in operation for the PtMo alloy. Preliminary results of an in situ powder X-ray diffraction (XRD) method which utilises synchrotron radiation (SR) and a curved image plate detector are also presented, using Pd/C as an example. The lattice expansion upon formation of the b-hydride is clearly observed. #
Journal of Physical Chemistry B, 2001
Structural and textural studies of a CuO/TiO 2 system modified by cerium oxide were conducted using Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N 2 absorption (BET specific surface area). The introduction of a minor amount of CeO 2 (Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 sample) resulted in a material with the maximum surface area value. The results of Raman spectroscopy revealed the presence of only two crystalline phases, TiO 2 anatase and CeO 2 cerianite, with well-dispersed copper species. TEM micrographs showed a trend toward smaller TiO 2 crystallites when the cerium oxide content was increased. The XPS analysis indicated the rise of a second peak in Ti 2p spectra with the increasing amount of CeO 2 located at higher binding energies than that due to the Ti 4+ in a tetragonal symmetry. The CuO/TiO 2 system modified by CeO 2 displayed a superior performance for methanol dehydrogenation than the copper catalyst supported only on TiO 2 or CeO 2 . and subjected to a 50 W ultrasonic vibration for 2 min. The mixture was allowed to rest for 24 h in a saturated atmosphere of isopropyl alcohol. , the resulting gel was dried at 383 K for 16 h and then calcined at 723 K for 16 h in air. The prepared powder CuO-CeO 2 /TiO 2 samples, called Ce 0.91 O 1.91 -Cu 0.09 , Ti 0.91 O 1.91 Cu 0.09 , Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 , Ce 0.27 Ti 0.64 O 1.91 -Cu 0.09 , and Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 , express the amount of each component as atomic fractions. TiO 2 and CeO 2 were used as structural references.
Journal of Solid State Electrochemistry, 2013
Micro-and mesoporous carbide-derived carbons synthesized from molybdenum and tungsten carbides were used as porous supports for a platinum catalyst. Synthesized materials were compared with commercial Vulcan XC72R conducting furnace black. The scanning electron microscopy, X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and low-temperature N 2 adsorption methods were applied to characterize the structure of catalysts prepared. The kinetics of oxygen electroreduction in 0.5 M H 2 SO 4 solution was studied using cyclic voltammetry and rotating disk electrode methods. The synthesized carbidederived carbons exhibited high specific surface area and narrow pore size distribution. The platinum catalyst was deposited onto the surface of a carbon support in the form of nanoparticles or agglomerates of nanoparticles. Comparison of carbide-derived carbons and Vulcan XC72R as a support showed that the catalysts prepared using carbide-derived carbons are more active towards oxygen electroreduction. It was shown that the structure of the carbon support has a great influence on the activity of the catalyst towards oxygen electroreduction.
Journal of The Electrochemical Society, 2005
An analysis of X-ray absorption spectroscopy ͑XAS͒ data ͓X-ray absorption near-edge structure ͑XANES͒ and extended X-ray absorption fine structure ͑EXAFS͔͒ at the Pt L 3 edge for Pt-M bimetallic materials ͑M = Co, Cr, Ni, Fe͒ and at the Co K edge for Pt-Co is reported for Pt-M/C electrodes in HClO 4 at different potentials. The XANES data are analyzed using the ⌬ method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode ͑RHE͒ representing a reference spectrum. These ⌬ data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt-M. This OH chemisorption, decreasing in the direction Pt Ͼ Pt-Ni Ͼ Pt-Co Ͼ Pt-Fe Ͼ Pt-Cr, is directly correlated with the previously reported fuel cell performance ͑electrocatalytic activities͒ of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt-Ni and Pt-Co clusters were more homogeneous with M atoms at the surface, while the Pt-Fe and Pt-Cr clusters had a "Pt skin." The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the presence of the Pt skin, or lateral interactions when M-OH groups exist on the surface.
Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review
Catalysts
The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox proper...
Frontiers in Energy, 2017
A method is described to determine the internal structure of electrocatalyst nanoparticles by in situ X-ray absorption spectroscopy (XAS). The nondestructive spectroscopic technique typically utilizing synchrotron radiation as the source measures changes in the X-ray absorption coefficient as a function of energy. The bulk technique has found its use for materials characterization in all scientific areas, including nanomaterials. The analysis of the internal structure of nanoparticles reveals interatomic distances and coordination numbers for each element, and their values and mutual relations indicate whether the elements form a homogeneous or heterogeneous mixture. The core-shell heterogeneous structure in which certain elements are predominantly located in the core, and others form the encapsulating shell is of particular importance in catalysis and electrocatalysis because it may reduce the amount of precious metals in nanoparticles by replacing the atoms in the core of nanoparticles with more abundant and cheaper alternatives. The examples of nanoparticle structures designed in the laboratory and the approach to model efficient catalysts through systematic analysis of XAS data in electrochemical systems consisting of two and three metals are also demonstrated.
Journal of the American Chemical Society, 2012
An in situ electrochemical X-ray absorption spectroscopy (XAS) cell has been fabricated that enables high oxygen flux to the working electrode by utilizing a thin poly(dimethylsiloxane) (PDMS) window. This cell design enables in situ XAS investigations of the oxygen reduction reaction (ORR) at high operating current densities greater than 1 mA in an oxygen-purged environment. When the cell was used to study the ORR for a Pt on carbon electrocatalyst, the data revealed a progressive evolution of the electronic structure of the metal clusters that is both potential-dependent and strongly current-dependent. The trends establish a direct correlation to d-state occupancies that directly tracks the character of the Pt−O bonding present.